1. Field of the Invention
The present invention relates to an injection apparatus of a die casting machine, in which injection operation is switched in three stages, low-speed injection process, high-speed injection process, and intensified-pressure injection process, by means of a piston-type accumulator.
2. Information Disclosure of the Related Art
In the die casting, after the molten metal is injected in the die under high-speed injection process, when the molten metal is solidified under the intensified-pressure injection process, it comes intimately into contact with the die so that its texture is very fine. Moreover, those drawbacks which are attributable to shrinkage by solidification are removed to ensure production of high-quality castings.
Thus, the injection apparatus of the die casting machine is constructed so that the injection speed can be changed for low- or high-speed injection process, the pressure on the molten metal in the cavity can be intensified after high-speed injection process (see Jpn. Pat. Appln. KOKOKU Publication. No. 54-18218).
Generally, in the injection apparatus of the die casting machine of this type, moreover, an accumulator is used in order to obtain the high-speed injection process. Accumulators conventionally used in hydraulic circuits of injection apparatuses may be classified into two types, piston type and bladder type.
FIG. 4 is a circuit diagram showing a hydraulic circuit of a direct-coupled injection apparatus of the piston type which utilizes a piston-type accumulator, among other conventional injection apparatuses having a function to switch injection operation.
In FIG. 4, numeral 60 denotes a piston-type accumulator for accumulating hydraulic oil. The accumulator 60 is connected to an injection cylinder section 63, which is formed of a duplex cylinder, through a high/low-pressure valve 61 and an intensified-pressure valve 62.
The high/low-pressure valve 61 includes a pilot check valve for switching the injection mode between low-speed injection and high-speed injection process, and feeds the hydraulic oil into a head-side cylinder chamber of a speed cylinder 64 of the injection cylinder section 63.
On the other hand, the intensified-pressure valve 62 feeds the hydraulic oil into a head-side cylinder chamber of a pressure cylinder 65 which is exclusively used to intensify the pressure of the injection cylinder section 63.
In the direct-coupled injection apparatus of the piston type constructed in this manner, the hydraulic oil from the accumulator 60 is fed through the high/low-pressure valve 61 into the speed cylinder 68 in a first low-speed injection process. In switching the injection mode to high-speed injection process, the high/low-pressure valve 61 is switched to a two-step opening so that the hydraulic oil is fed at a higher flow rate into the speed cylinder 64. In intensifying the pressure on the molten metal in the cavity, the intensified-pressure valve 62 is opened to allow the hydraulic oil to be fed from the accumulator 60 into the pressure cylinder 65, whereupon the piston advances. Thus, the pressure on the molten metal can be intensified, while a pilot-check valve of the high/low-pressure valve 61 is closed to prevent the hydraulic oil from reversing into the high/low-pressure valve 61.
FIG. 5 is a hydraulic circuit diagram showing a hydraulic circuit of an injection apparatus of the single-acting accumulator type which utilizes a bladder-type accumulator.
This injection apparatus is provided with first and second rubber accumulators 46 and 47. The pressure of hydraulic oil accumulated in the first accumulator 46 is adjusted for low- or high-speed injection process, while the pressure of hydraulic oil accumulated in the second accumulator 47 is increased for intensified-pressure injection process.
A simplex cylinder is used as an injection cylinder 48, and pressure oil is supplied from the first accumulator 46 to the head side of the cylinder 48 through an intensified-pressure valve 49. On the other hand, pressure oil is supplied from the second accumulator 47 to the injection cylinder 48 through an intensified-pressure valve 50.
In executing low-speed injection in the injection apparatus constructed in this manner, the operating speed of the injection cylinder 48 is adjusted to a low speed injection process by restricting the flow rate of the hydraulic oil on the outlet side of the cylinder 48 by means of a low-speed valve 51. In switching the injection mode from the low-speed injection process to the high-speed injection process, meter-out control is effected such that the pilot pressure is applied to a check valve 52 to open it, thereby increasing the flow rate of the hydraulic oil on the outlet side of the injection cylinder 48. In starting the intensified-pressure injection, the pilot valve acting on the check valve of the intensified-pressure valve 50 is released to open the valve 50, thereby changing the control mode to intensified-pressure injection control. Numeral 53 denotes a relief valve through which the hydraulic oil is delivered from the injection cylinder 48.
However, the prior art piston-type injection apparatus requires use of the exclusive cylinder 65 for intensified pressure besides the intensified-pressure cylinder 64, so that the injection cylinder section 63 is inevitably bulky as a whole.
According to the injection cylinder of the single-acting accumulator type, on the other hand, the pressure of the hydraulic oil must be controlled by means of the accumulators 46 and 47 are provided for the low- or high-speed injection process and the intensified-pressure injection process, thus requiring an additional installation space for the accumulators.